A filler is added, for example, in order to improve mechanical properties such as strength, or thermal properties of a polymer film. In the recent trend of miniaturization of electrical equipment, there is a thinning trend in the field of polymer films. The thinner a polymer film is, the finer and shorter a filler added to the polymer film must be. This is because if the filler is thicker or longer than the thickness of the polymer film, the filler is likely to protrude from the polymer film, and therefore, many practical problems arise.
As an inorganic short fiber applicable to such a filler, the applicant of the present application has proposed “an inorganic short fiber, which is mainly composed of an inorganic component, wherein the average fiber diameter is 2 μm or less, the average fiber length is 20 mm or less, and the ratio (Ld/La) of the standard deviation (Ld) of the fiber length to the average fiber length (La) of the inorganic short fiber is 0.3 or less.” (Patent literature 1). The average fiber diameter of the inorganic short fiber is small, and thus, it was considered to be applicable to a thin-film polymer film. Although this document exemplifies conventional fiber cutting machines, such as a guillotine-type cutting machine and a rotary cutter-type cutting machine, as a method of manufacturing the inorganic short fiber, even if these conventional fiber cutting machines were used, it was extremely difficult to obtain inorganic short fibers having a short fiber length of 200 μm or less, as described in the Examples of the document, due to the mechanism of the machines, and it was difficult to use the fibers as a filler for thinner polymer films. In addition, since these cutting machines use a metal blade, there was a problem wherein the metal blade was damaged at the time of cutting, and a piece of metal was mixed in with inorganic short fibers or an inorganic fiber aggregate. This problem was remarkable in the case of manufacturing short fibers consisting of a hard inorganic material such as alumina, which was also used as an abrasive.
A ball mill, a mortar, and the like are known as devices for crushing fibers. However, even if these crushing devices were used, fibers could not be sufficiently crushed to obtain fine fibers having an average fiber diameter of 2 μm or less, or even if fibers could be crushed, short fibers in which the fiber length were uniform could not be prepared.
On the other hand, a filler is added in order to improve mechanical properties such as impact resistance and tensile strength, or thermal properties such as thermal extension suppression of a transparent resin sheet. In such a filler, in addition to imparting these properties, it is necessary to not impair the transparency of the transparent resin sheet. Therefore, the refractive index of the filler is adjusted to the refractive index of the transparent resin sheet.
For example, “a filler for polycarbonate resin consisting of a glass component, wherein silicon dioxide (SiO2), aluminum oxide (Al2O3), and titanium oxide (TiO2) are contained as essential components; at least one of lithium oxide (Li2O), sodium oxide (Na2O), and potassium oxide (K2O) is contained; and the contents of the components, as represented by mass %, are 45≤SiO2≤65, 0.1≤Al2O3≤15, 9≤(Li2O+Na2O+K2O)≤25, and 15≤TiO2≤25.” (Patent literature 2) has been proposed. However, this filler could not be applied to resins other than polycarbonate resins. In addition to this problem, it was considered that the adjusting of the refractive index was difficult, because it contained at least four components.
In connection with this, the applicant of the present application did not intend to adjust the refractive index, but disclosed that a silica stock solution and an alumina stock solution were mixed to prepare a sol solution, the sol solution was fiberized by electrospinning, and the fibers were dried and sintered to produce an inorganic structure consisting of silica-alumina sintered ultrafine long fibers (Patent literature 3). It was examined whether or not the refractive index could be controlled utilizing this technique, but the fine adjustment of the refractive index was difficult, and it was difficult to obtain silica-alumina sintered ultrafine long fibers having a uniform refractive index.